For QCD one also has a low-energy EFT in order to describe the interactions between pions, which is called Chiral Perturbation Theory (ChPT) [1]}, [2]}. In the \(SU(2)\)  chiral limit, with zero \(u\)  and \(d\)  quark masses, the pions are Goldstone bosons and their interactions can also be organized in terms of operators with an increasing number of derivatives, \(i\partial _\mu \sim p_\mu \) . For this EFT the unitary cutoff \(\Lambda _U=4\pi f_\pi \simeq 1.2\)  GeV, with \(f_\pi \)  the weak pion decay constant, while the cutoff associated with the higher states in QCD is \(\Lambda =M_\rho \simeq 0.8\)  GeV, with \(M_\rho \)  the mass of the \(\rho (770)\)  [3]}. Below such energies, even more when taking the Mandelstam variable \(s\) , one finds the resonance \(f_0(500)\)  aka \(\sigma \)  [3]}, such that \(m_\sigma ^2/(4\pi f_\pi )^2\approx 0.14\) . The smallness of this number makes possible to afford the study of this resonance by applying ChPT to a properly chosen interaction kernel by implementing so-called unitarization techniques [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, see Ref. [13]} for a recent review. Schematically the unitarization techniques allow to resum the set of diagrams depicted in Fig.  REF
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